Superheterodyne receiver



March 9, 1937. J. K. JOHNSON 7 2,073,345

SUPERHETERQDYNE RECEIVER Original Filed May 22, 1934 DETECTOR AND AUDIOFREQUENCY AMPLIFIER AND LOUD SPEAKER INTERMEDI TE FREQUEIFCY AMPLIFIERJHN KELLY JOHNSON, 0 BY ATTORNEY.

Patented 9, 1931.

UNITED STATES.

PATENT orrlce 2,013,315 surnnnn'ranonm nncaivsn John Kelly Johnson,Kcnilworth, IlL, signer to Hazeltine Corporation, Jersey City, N. J.,-aoerporation of Delaware Original application May 22, 19:4, Serial No.726,931. Divided and this application January 14, 1936, Serial No.59,021

6 Claims. '(CL 250 -36) This invention relates to oscillation circuitstunable over a range of frequencies by an adjustable reactance thereofto select a desired 'fre-.

quency of the range, .and more particularly to such circuits includingprovisions for independently adjusting the resonant frequency thereofrelative to the selected frequency over a small fraction of the tuningrange.

This application is a division of my copending 10 application Serial No.726,931, filed May 22, 1934.

In an oscillation circuit containing inductance and capacitance, one ofwhich is adjustable for tuning the circuit over a desired range, it issometimes desired to adjust the resonant frequency thereof, relative tothe frequency selected by the quency to adjust the selectivity of thesystem, to

which feature my above-mentioned copending application is directed. Suchadjustment of. the

oscillation frequency may be effected by provid- 5 ing in connectionwith the circuit adjustable auxiliary impedance means. However, thereactance ofthe adjustable tuningelement of the circuit varies withinwide limits 'as the element is adjusted to tune the circuit over itsrange. The effectiveness of the auxiliary impedance means in adjustingthe resonant frequency of circuit is ordinarily dependent on therelative magnitude of its impedance and those of the main reactanceelements of the circuit. Therefore, the effectiveness of the auxiliaryimpedance means tends to vary in accordance with the frequency to whichthe eircuit is tuned and, in order to obtain equal adjustments of theresonant frequency of circuit relative to different selectedfrequencies, different degrees of adjustment of the auxiliary meanswould ordinarily be required. It is highly desirable, however, thatequal adjustments of said auxiliary impedance means willeifect equalad-- justments of the resonant frequency of the circuit for allfrequencies of the range through I which the circuit may be tuned. J)

It is the primary object, therefore, of the present invention to providea circuit arrangement w by. the resonant frequency of an oscillato anyselected frequency tion circuit, tunable within a predetermined range offrequencies may be adjusted relative to the selected frequency, andindependently of the tuning means over a small fraction v v of therange, ,equal adjustmentsof the adjusting means effecting equal changesof the resonant frequency of the circuit relative to the selectedfrequency for all frequencies of the range. For a better understandingof my -in-- vention, together with other and further objects thereof,reference is had to the following description taken in connection withthe accompanying drawing. and its scope will be pointed out in theappended claims.

In accordance with the present invention, an oscillation circuit isprovided with auxiliary ad-' justable impedance means, independent ofthe.

tuning element of the circuit, and coupled to the circuit to adjust theresonant frequency there-- .of. The auxiliary means and its coupling tothe circuit are so proportioned" and arranged that the effectivenessthereof is substantially constant and independent of the frequency towhich the circuit is tuned by its tuning element.

In apreferred embodiment of the invention, the oscillation circuit iscomprised in an oscillation system, the auxiliary adjustable impedancemeans comp dses a variable condenser connected across a port-ion of thecapacitance of the circuit through at least a. portion ofthe inductanceof the system, and the auxiliary'condenser and .its'

connections are so proportloned'and arranged as to provide the desiredconstancy in effectiveness of the auxiliary variable condenserfor anyselected frequency within the operating range of the system. I

In the accompanying drawing Fig. 1 is a circuit diagram of a completesuperheterodyne receiver,

partly schematic, illustrating the present invention applied to thelocal oscillator to adjust the selectivity ofthe receiver, as disclosedin my aforesaid copending application, while Fig. 2'

shows graphically the relation between the several frequencies of thesystem.

Referring nowtorig. 1, there is illustrated a superheterodyne receivercomprising, in cascade,

an antenna and ground: system "-1 l; a radiofrequencyamplifier ii, anoscillator-modulator tube If; an intermediate-frequency amplifier I4;and a detector, audio-frequency amplifier and loud+speaker II. Thedevices i2, i4 and I! are indicated only schematically, since theyconsti-' tute no part of this invention and may be of any conventionaldesign.

The oscillator-modulator tube ll is of the hexode type and may take theform disclosed in Patent No. 2,015,327 issued to Hamid A. Wheeler onSeptember 24, 1935. The tube contains a signal input grid it coupled tothe radio-frequency amplifier I! through a radio-frequency transformerll, the secondary .coil of which is tunable to signal channels of thebroadcast range by a variable condenser l8.

There is associated with tube l8 an oscillation system l9, embodying thepresent invention, which includes a coil 20 connected from an innerscreen 2| to an inner grid 22 through a condenser 28. A condenser 24 isconnected betweenground and an intermediate point 25 of the coil. Asource of positive operating voltage, designated +18, is connectedthrough a resistor 26 to the point 25 for supplying a proper directvoltage to the screen 2|. A variable condenser 21 is connected betweenground and' the upper end of coil 28. .The condensers 24 and 21 and theportion of coil above point constitute a resonant circuit whichdetermines the frequency of the oscillation system, this frequency beingvariable by means of condenser 21. The oscillator-modulator circuitjustdescribed is similar to those 20 which are more fully described inUnited States Letters Patent No. 1,958,027, granted May 8,

1934, upon the application of Harold A. Wheeler.

' In accordance with the present invention, there is connected arelatively small variable con- 25 denser 28 between ground and a secondintermediate point 29 of coil 20 which is outside the resonantfrequency-determining circuit. By reason of its connection to ground,condenser 28 is connected across condenser 24 and a portion 30 of coil20 which is outside the resonant circuit.-

The function and operation of condenser 28 will be more fully explainedsubsequently.

Modulation occurs between the received signal.

frequency impressed on, grid I8 and the local .35 oscillator frequencyin a manner which is fully described in the above-mentioned Patent No.1,958,027. The products of modulation at the circuit of anode 30 includethe intermediate-frequency band which is the difference between thelocal oscillator frequency and the frequencies of the received signalchannel, For the purpose of maintaining a constant frequency differenceand therefore a quency, the variable condensers l8 and 21 are gangedtogether for simultaneous operation by means of a unicontrol deviceindicated generally by U.

A coupling system ,3l connected between the anode of tube l8 and theinput of the intermediate-frequency amplifier I4 is tuned to pass afixed intermediate-frequency band.

The frequency band selected and passed by the tuned coupling system 3|is shown graphically in Fig. 2. In this figure, the intermediate carrierI 5 frequency is represented by the vertical line A which is taken askilocycles in this case. The coupling system is adjusted to select andpass an intermediate-frequency channel band 8 kilocycles wide, from 171to 179 kilocycles, these two edges of the selected band being indicatedby the broken lines B and C,.respectively.

The function of the condenser 28 associated with the oscillator circuitis to shift the oscillator frequency in either direction by any desiredamount up to the width of a side band,-without altering theradio-frequency channel selected. Since the side-band width for the caseunder consideration is 4 kilocycles, this should be the limit offrequency shift due to condenser 28. It will be noted that this range ofadjustment is a very small fraction' of the tuning range of the system,which is usually of the order of 1000 kilocycles to cover the generalbroadcast. band. If the oscillator frequency is higher than the receivedsignal 7 frequency, as is usually the case, anydecrease in constantintermediate carrier fre-' the oscillator frequency without an attendantchange in the selected radio frequency, will shift the intermediatecarrier frequency downward by a like amount, and vice versa. In theexample under consideration, therefore, the oscillator frequency will beconsidered to be 175 kilocycles higher than the selected signalfrequency, when condenser 28 is in its neutral or normal position. Thedotted line D in Fig. 2 shows the location of the intermediate carrierfrequency when the oscillator frequency is shifted downward by 3kilocycles from normal. It is contemplated that the intermediatefrequency represented by line D may be shifted at will to any positionbetween the limits of lines B and C.

Since the position of the intermediate-frequencyband selected by thecoupling system 3| and the intermediate-frequency amplifier l4 remainsfixed, such a shift of the derived inter- -mediate carrier frequency hasthe effect of cutting off the outer frequencies of one side band and ofadding on to the outer frequencies of the other side band by a likeamount. For the amount of shift illustrated in Fig. 2, there remains inthe lower side band only the frequencies betweenl'll and 172 kilocycles,corresponding to audio frequencies from 0 to 1000 cycles.. The lowerintermediate-frequency side-band frequencies corresponding to audiofrequencies of '1 to 4 kilocycles have not been selected. The

upper intermediate-frequency side band, however, is now 7, instead of 4,kilocycles wide; for it now extends from 172 to 1'79 kilocycles. Hence,instead of transmitting onlyf frequencies corresponding to 0 to 4kilocycles ofaudio frequency, the selecting system now transmitsfrequencies corresponding to 0 to 7 kilocycles of audio frequency. A

In the manner just described, the audio-frequency fidelity is improvedwithout increasing the transmitted band width; although it should benoted that at the same time frequencies corresponding to low audiofrequencies (0 to 1000 in this case) are transmitted with betterefliciency than the frequencies corresponding to higher. audiofrequencies. The reason for this is that frequencies corresponding tolow audio frequencies are transmitted by'both side bands.

This inequality of transmission can be compensated to a considerableextent by a properly designed equalizing network in the audio-frequencyamplifier.

It is noted that the arrangement of condenser 28 is particularlyadvantageous. By reason of its connection across at least some of thecapacitance of the frequency-determining circuit and across inductancewhich is outside the frequencydetermining circuit, equal adjustments orvariations of the capacitance of condenser 28 produce equal frequencyshifts or adjustments at all frequencies of the tuning range. Such wouldnot be the case if condenser 28 were connected across inductance aloneor capacitance alone.

As is well understood, the condenser 24 has a maximum effect in thecircuit with maximum adjustment when the condenser 27 is adjusted to itsmaximum value to provide a minimum reactance for tuning the circuit to alower frequency of the-range, and has a. minimum effect when condenser28 were connected only across the condenser 24 so that its effect wouldvary with frequency in the same manner as the condenser 24,

auras uencies inan opposite sense to that of the condenser 28, theresultant capacitive reactance of the auxiliary frequency adjustingcircuit in parallel with the condenser is modified so that equaladjustments of condenser 2. effect equal changes in the-resonantfrequency of the circuit at all frequenciesof its tuning range.

Since the intermediate carrier frequency may be shifted in eitherdirection. the direction chosen for the shiftwill ordinarily be oppositethat in whichaninterferingsiznalmayhappentober present. 7 Such ashift'will tune out such an interfering signal at the same time that thefidelity is being improved.

. While have describedwhat I at present consider the preferredembodiment of my invention,

itwillbeobvious tothoseskilledinthe artthat ,varlous changesandmodifications may be made therein without departing from my invention,and

- I, therefore, aimin the appendedclaimsto cover alls'uch changes andmodifications as fall within thetruespirltandscopeofmyinventim What isclaimed is:

1. An circuit tunable overarange loffrequenciesandincludinganadjmtablereactanceelementfortuningsaidcircuittoaselectedfrequency .of said range, and meals adjustable independently of saidtuning element for adjusting the resonant frequency of said circuitrelative tosaidselectedfrequency'overasmallfraction of said range,auxiliary impedance means wlm edtosaid circuit, the im- 'pedanceofsaidaimpedancemeans'being sorelatedtoitscouplingtosaidcircuitthatcqualadjustmentsthereofqfectequalad iustments oftheresmiantfrequencyof-saidclrcuit-forallofsaidrange, 2. Anoscillationcircuit=tunable over a rangeoffrequenciesandincluiiinganadjustablereactance elementfortuningsaidcircidt-toaselected frequency" of said range, and means adjustableindependently of said=tlmlng element for adjusting the resonantfrequency of said circuit rela-.tivetosaidselectedfrequencyoverasmallfrac-- tion ofsaidrangecomprisingvariabiecapaci- 66 tance means coupled to said circuit; thereactance of said'variabie capacitance-means being so relatedtoitscoilpling-to that equal adjustments thereof elect-"substantiallyequal adjimtments-of. the resmanttrequeney e1.

said'circuit forallfrequencies afraid 3. An oscillationcircuittunabiecverarange.

offrequenciesandincluding'adjlltablereactance said circuit for allfrequencies of saidrange.

' frequencies of said range.

of said auxiliary impedance means being adjustable and said means beingconnected in parallel with at least a part of the reactance of said.circuit and the reactance of said auxiliary means being so related tothat of said part of said circuit with which it is connected inpa'r'allel that equal adjustments of said adjustable part of saidauxiliary impedance means eflects substantially equal adjustments of theresonant frequency of 4. An oscillation circuit tunable over a range offrequencies and including inductance means and a variable condensereffectively connected across at least a portion of said inductance meansfor tuning said circuit to a selected frequency of said range, andmeansadjustable independently of said'variable condenser for adjustingthe'resonant frequency of said circuit relative to said selectedfrequency over a small fraction of said range, comprising an auxiliaryadjustable condenser connected in parallel with at least a portion ofsaid circuit including at least a portion of the capacitance of saidcircuit-and atleast a portion of said inductance, the reactance of saidauxiliary condenser being so related to that of-the portion of saidcircuit with which it is connected in parallel that equal adjustments ofsaid auxiliary condenser eifect substantially equal adjustments of 'theresonant frequency of said circuit for all frequencies of said range.

5. An oscillation system comprising a vacuum tubeand anoscillationcircuit, tunable over-a range of frequencies,- associatedwithsaid tube and including an adjustable reactance element for tuningsaldcircuit to a selected frequency of said range. and meansadjustable-independently of said tuning element for adjusting theresonant frequency of said circuit relative to said selectedfrequencyover asmall fraction of said range,

comprising auxiliary adjustable impedance means coupled to said circuit.the impedance ofsaid auxiliaryimpedance means being so related to itscoupling to said circuit that equal adjustments thereof effectsubstantially equal adjustments of the frequency generated by saidsystem for all 6,. An oscillation circuit timable over offrequencies andincluding inductance. means and variable capacitance means effectivelyconnected across two points of said inductance for tuning said circuitto a selected frequency of said misc, s'aid capacitance means having anintermediate connection point, 'andmeans adjustable independently ofsaid variable capacitance means for adjusting the resonantfrequencyofsaid circult relativeto said selected frequency over a small fractionof said range, comprising an auxa range iliary variable condenserconnected between a third point of said inductance and said intermediatepoint of said capacitance means.. the

reactance of said auxiliary condenser being 'so mated to that of theportion of-saidcircuit.

across which it is connected that equal adjustments of said auxiliarycondenser efl'ect substantially equal adjlutments of the resonant fre--quency of said circuit for all frequencies of said some min! Johnson.

